Technical Field
The present disclosure relates to a high-efficiency energy harvesting interface and to a corresponding energy harvesting system.
Description of the Related Art
As is known, systems for harvesting (or scavenging) energy from mechanical or environmental energy sources arouse considerable interest in a wide range of technological fields, for example in the field of portable electronic devices or in the automotive field.
Typically, energy harvesting systems are designed to harvest, store, and transfer energy generated by mechanical or environmental sources to a generic electrical load, which may be supplied, or, in the case of a battery, recharged. These systems thus enable production of electronic apparatuses that operate without a battery, or with a considerable increase in the duration of batteries in the case of apparatuses provided therewith.
For harvesting environmental energy, solar or thermoelectric generators may be used, which convert solar energy and thermal energy, respectively, into electrical energy.
FIG. 1 shows schematically and by functional blocks, an energy harvesting system of a known type.
The energy harvesting system 1 comprises a transducer 2, for example a photovoltaic or thermoelectric generator that includes a plurality of cells (of a known type, not described in detail herein), which converts solar energy or thermal energy into electrical energy, typically into a DC voltage or, in any case, into a voltage that varies slowly in time (with respect to the electrical constants of the circuit), generating a transduction signal VTRANSD.
The energy harvesting system 1 further comprises a harvesting interface 4, designed to provide a condition of coupling with the transducer 2 of the MPPT (Maximum Power Point Tracking) type, in order to maximize extraction of power. The harvesting interface 4 is configured to receive at input the transduction signal VTRANSD generated by the transducer 2 and supply at output a harvesting signal VINDCDC.
The energy harvesting system 1 further comprises: a storage capacitor 5, which is connected to the output of the harvesting interface 4 and receives the harvesting signal VINDCDC, which determines charging thereof and consequent storage of energy; and a DC-DC converter 6, connected to the storage capacitor 5 for receiving at input the stored electrical energy and generating at output a regulated signal VREG, with an appropriate value so that it may be supplied to an electrical load 8, for its supply or its recharging.
The global efficiency ηTOT of the energy harvesting system 1 is given by the expression:ηTOT=ηTRANSD·ηMPPT·ηDCDC where: ηTRANSD is the efficiency of the transducer 2, indicating the amount of environmental energy, effectively converted by the transducer 2 into electrical energy; ηMPPT is the efficiency of the harvesting interface 4, indicating the amount of converted electrical energy that is effectively used for charging the storage capacitor 5; and ηDCDC is the efficiency of the DC-DC converter 6.
In particular, the efficiency ηMPPT of the harvesting interface 4 indicates the ratio between the power effectively transferred onto the storage capacitor 5 and the maximum power that could theoretically be supplied, PMAX.
In detail, this efficiency ηMPPT is given by the following expression:ηMPPT=ηCOUPLE·ηLOSS where ηCOUPLE is the coupling factor between the transducer 2 and the harvesting interface 4 (indicating the impedance matching between the same transducer 2 and the harvesting interface 4), and ηLOSS is the loss of power due to consumption by the harvesting interface 4.
It has been shown that, in the case of a thermoelectric cell, which may be represented schematically, as illustrated in FIG. 2a, as an equivalent voltage generator VOC connected to a series resistance RTEG, the efficiency ηMPPT is maximized in the case where the transduction signal VTRANSD has an optimized value VMPPT equal to VOC/2 (i.e., a value equal to one half of the load-less, or open-circuit, voltage supplied by the corresponding equivalent voltage generator).
Likewise, in the case of a photovoltaic cell, which may be represented schematically, as illustrated in FIG. 2b, as an equivalent current generator IPV connected in parallel to a diode DPV (in the figure, the equivalent series resistance of the generator is not represented), the efficiency ηMPPT is maximized in the case where the transduction signal VTRANSD has an optimized value VMPPT comprised between 0.75·VOC and 0.9·VOC (according to the constructional parameters of the photovoltaic cell and the material of which it is made), for example equal to 0.8·VOC, where VOC is once again the open-circuit voltage supplied by the photovoltaic cell.
It is consequently required that the harvesting interface 4 of the energy harvesting system 1 be configured in such a way that the transducer 2 operates in, or around, a working point that ensures the aforesaid condition of maximum efficiency.
For this purpose, a wide range of circuit configurations have been proposed for providing the harvesting interface 4.
For instance, in the document entitled “A Seamless Mode Transfer Maximum Power Point Tracking Controller for Thermoelectric Generator Applications” by Rae-Young Kim, Jih-Sheng Lai, IEEE Transactions on Power Electronics, vol. 23, No. 5, September 2008, an interface circuit has been proposed, comprising a dual voltage conversion stage, formed by the cascade of a boost converter and a buck converter, the latter being designed to regulate the value of the output voltage. Tracking of the MPPT condition is obtained with a continuous-time control of the duty cycle of the boost converter.
The present Applicant has, however, realized that this solution involves a high power consumption, which is due to the fact that the control is of a continuous-time type, which does not render it suited to energy harvesting applications. Further, this solution does not prove flexible, being suited only to a specific type of transducer and to precise values of the electrical parameters associated thereto, further depending upon the tolerance in the values assumed by the same electrical parameters. In general, this solution also involves a large number of external components, which may not be made with integrated technology.
Another possible circuit implementation is described in the document entitled “Thermoelectric Energy Harvesting with 1 mV Low Input Voltage and 390 nA Quiescent Current for 99.6% Maximum Power Point Tracking” by Chao-Jen Huang, Wei-Chung Chen, Chia-Lung Ni, Ke-Horng Chen, Chien-Chun Lu, Yuan-Hua Chu, and Ming-Ching Kuo, 38th European Solid-State Circuits Conference (ESSCIRC), September 2012. This solution envisages a boost converter and a continuous-time algorithm, the so-called perturbation and observation algorithm, to achieve the MPPT condition; in particular, the duty cycle of the converter is perturbed, and the trend of the output voltage is measured: the MPPT condition corresponds to a maximum positive trend.
The present Applicant has, however, realized that also this solution has some disadvantages, amongst which: a high power consumption, intrinsic in a continuous-time perturbation and observation algorithm, which renders it difficult to use in energy harvesting applications; and a poor efficiency, when combined to a low-power transducer.
The document entitled “A Coreless Maximum Power Point Tracking Circuit of Thermoelectric Generators for Battery Charging Systems”, by S. Cho, N. Kim, S. Park, S. Kim, IEEE Asian Solid-State Circuits Conference, Nov. 8-10, 2010, Beijing, China, describes yet a further solution for providing the harvesting interface. This solution envisages two conversion stages, with the cascade of a boost conversion stage and a buck conversion stage, the latter for regulation of the output voltage; the MPPT condition is achieved by a control of the switch in the boost stage.
The present Applicant has realized that also this solution, albeit presenting a simpler algorithm to achieve the MPPT condition, does not have a high efficiency, on account of the presence of two conversion stages. Further, also this solution requires a large number of external components, not made with integrated technology.
The subject matter discussed in the Background section is not necessarily prior art and should not be assumed to be prior art merely as a result of its discussion in the Background section. Along these lines, any recognition of problems in the prior art discussed in Background section or associated with such subject matter should not be treated as prior art unless expressly stated to be prior art. Instead, the discussion of any subject matter in the Background section should be treated as part of the inventor's approach to the particular problem, which in and of itself may also be inventive.